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Repelling an ant invasion

Q: Our new home has fruit trees, vines, and a large vegetable garden. There are also billions of ants. I’ve tried cinnamon, lemon pieces, ant stakes, ant traps, Tree Tanglefoot, and insecticidal powder, but I don’t see their population diminishing. 

Kelly Wood, Menlo Park, CA

A: Eddie Dunbar, Insect Hotline Operator, University of California, Berkeley Cooperative Extension Service, replies: Your problem is undoubtedly the Argentine ant, which is our most common urban ant pest and by far the most difficult ant to control. This ant has a sweet tooth and frequently feeds on citrus, cherry laurels, figs, bamboos, and many species of pine. It also likes honeydew and will attack plants that are infested with honeydew-producing insect pests—aphids, mealybugs, and soft scale. In fact, Argentine ants have even been known to “farm” these pests and protect them from their natural predators.

Controlling any pest requires identifying and limiting access to food. Sticky barriers such as Tanglefoot and Stickem placed around a tree trunk prevent the ants from reaching their food source. Also, keeping the plants healthy will help them resist infestations of honeydew-producing pests. 

Spraying the ants with water simply encourages the ants to move their eggs and larvae to a safer location. Killing foraging worker ants with pesticide sprays also provides little relief because an Argentine ant colony can survive with a mere 1 percent of its worker-ant force. 

The best way to control Argentine ants is with ant baits, particularly those containing hydramethylnon. However, baits will only be effective when the preferred natural-food source is eliminated. Timing and placement of baits are also critical. Reducing the ant population in spring will head off problems later in the year. Place the bait directly in the ant trails; their poor eyesight and limited olfactory senses won’t guide them to it.

In time, these ants may also invade your home. As nectars and other sweet-stuffs are washed from plants with winter rains, the ants will turn their attention to indoor food sources, so spring is the time to renew your baiting efforts.

From Fine Gardening 51, pp. 20